Geochemical exploration method by infrared analysis of soil extract solutions



March 1950 N. P. STEVENS 2,500,

GEOCHEMICAL EXPLORATION METHOD BY INFRARED ANALYSIS OF SOIL EXTRACT SOLUTIONS Filed March 28, 1945 2 Sheets-Sheet 1 2.0 2.9 .10 3.1 3.2 3.3 3.4 3.5 3.5 3.7 3.8 .29 4.0 WAVE LENGTH IN MICRON;

FIG. 1

Nelson P Sievens INVENTOR March 14, 1950 N. P. STEVENS 2,500,213

GEOCHEMICAL EXPLORATION METHOD INFRARED ANALYSIS OF son. EXTRACT so IONS Filed Marh 28, 1945 2 sheets-sheet 2 Nag 2..

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GEOCHEMICAL EXP oRATm E'rHoD BY INFRARED ANALYSIS OF son. EXTRACT SOLUTIONS Nelson P. Stevens, Dallas, Tex., assignor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a

corporation of New York Application March 28, 1945, Serial No. 585,259

8 Claims.

This invention relates to geochemical exploration for valuable mineral deposits in the earth, such as petroleum oil and natural gas. More particularly the invention relates to a method of locating such deposits by means of soil analysis.

It has been found that the presence of small "quantities of hydrocarbon materials in the near surface zones of the earth is often indicative of the existence of a deep-seated petroliferous deposit. The significant hydrocarbons may occur in the soil in the form of solids, liquids, or gases,

which are generally considered to have been produced from emanations originating in the deep-lying deposits. The present invention is particularly concerned with the measurement of those soil hydrocarbons which normally exist as liquids, or in the form of solid or semi-solid materials, which are essentially waxlike in their appearance and properties.

To explain the presence of the significant hydrocarbons in the soil and their relation to petroleum deposits the theory is generally held that they are derived from light diffusible substances, such as hydrocarbon gases, which continuously migrate from deep-seated deposits towards the earths surface and which upon reaching the near surface zones undergo chemical reaction, such as oxidation and/or polymerization, to form the less volatile liquid and waxlike compounds, which are deposited and accumulated in the rock pores of the soil.

The concentration of the significant hydrocarbons in the soil is generally very low, amounting in most instances to only a few hundredths of a percent by weight. In fact it has been my own experience in conducting numerous surveys in which the hydrocarbon concentrations were determined by the method of the present invention, hereinafter described, that the significant quantities often amount to only ten parts per million or less, by weight. It will therefore be appreciated that in order for the indications obtained in any soil hydrocarbon survey to be completely dependable, the analytical technique used therein should be capable of accurately measuring these minute concentrations of hydrocarbons in the soil. The methods presently available however, especially those which utilize gravimetric techniques, are not capable of accurately measuring such small amounts of hydrocarbons in the soil. Furthermore, these methods are for the most part tedious and time consuming, which is disadvantageous since it tends to greatly increase the cost of a soil hydrocarbon survey.

It is the prime object of the present invention to provide new and improved means of geochemical prospecting for petroleum. It is a further object to provide an improved method for the measurement of soil hydrocarbons present in quantities of only a few parts per million. Another object is to provide a rapid, precise method of measuring these minute quantities of hydrocarbons in the soil. Other and further objects and advantages of the invention will be apparent from the following description thereof.

Essentially, my invention comprises the steps of collecting soil samples from spaced points in a prospect area, extracting the samples with a suitable hydrocarbon solvent, such as carbon tetrachloride, determining the hydrocarbon contents of the extract solutions thus obtained by means of infra-red analysis and correlating the analytical results with the various sampling point locations in order to derive indications of the presence and/or location of the deep-seated petroliferous deposits.

In the practice of my invention a number of soil samples are collected from points set out in the prospect area according to some definite plan or pattern. For example, the sampling points may be spaced at intervals of say of a mile along a traverse, or along a series of traverses. Sampling is generally conducted at a depth a few feet below the surface, say, for example, about 6 feet. However, where it is practicable to do so, I prefer to recover all of the samples in any one survey from the same earth stratum, the sampling depth being varied accordingly, if necessary. I have found that when this procedure is followed, the survey results are more reliable since in this way the characteristics of the soil samples, such as porosity and/or absorptive ability, which are apt to affect their hydrocarbon contents, are substantially constant and errors which would otherwise be introduced into the analytical results are avoided.

The recovered samples are air dried and pulverized, and then extracted with a suitable hydrocarbon solvent. It is important that the solvent selected for the extraction of the soil samples exhibit a high transmission to infra-red radiation, particularly over the range where the hydrocarbons to be analyzed exhibit their characteristic absorption bands. Carbon tetrachloride for example is such a solvent, and I have found it to be highly satisfactory for this purpose. Other solvents have been found suitable where such solvents are free from carbon-hydrogen bonds, such as ethylene tetrachloride, carbon disulphide and the like.

The extraction of the samples may be conveniently carried out by the use of the well known Soxhlet extraction apparatus. By this method, an accurately weighed portion of sample, for instance 150 grams, is placed in a fat-free extraction thimble, which in turn is placed in the Soxhlet extractor. Suificient carbon tetrachloride, required for normal operation of the extraction apparatus, viz. about 175 00., is then added to the extractor. The extractor is operated by regulating the heating thereof so that the refluxing extraction solvent is siphoned from the soil sample once every ten minutes. The extraction is continued at this rate for a period of 4 hours, after which the apparatus is allowed to cool and the extract solution removed. Since the volume of the solvent will have been substantially decreased by absorption in the body of the soil sample, sufiicient solvent is finally added to bring its volume up to some standard value, for example 100 cc. It will be appreciated that thisequalization of the volumes of the different extract solutions acts to facilitate the interpretation of the analytical results obtained in the subsequent infra-red analysis step of the invention, since in this way a direct comparison of the infra-red absorptive abilities of the different samples is permissible.

Before the extract solutions are analysed however, it is important that all traces of water be removed therefrom. Accordingly, it is recommended that a small amount of a solid desiccant, such as barium oxide, be added to each of the extract solutions and that the solutions be allowed to stand for a short time. The solutions are then transferred to an infra-red absorption cell for analysis.

The determination of the hydrocarbon contents of the extracted soil samples by means of infrared spectroscopy in accordance with my invention is based upon the well known fact that hydrocarbons have the ability to absorb energ from infra-red rays. It hasbeen observed heretofore that the absorbing power of different hydrocarbon compounds as regards infra-red radiation is selective in character. Thus, a particular hydrocarbon will exhibit greater or less absorptive power depending on the particular wave length of infra-red radiation being transmitted therethrough.

By making numerous extractions of soil samples and testing the absorptive power of the resulting extract solutions over the entire range of the infra-red spectrum, I have determined that the indicating soil hydrocarbons consistently exhibit their strongest absorptive effects in the spectral range of from 2.8 to 4.0,. For this reason and because carbon tetrachlorideexhibits very high transmission of infra-red rays-within the range of these wave lengths, I have selected this portion of thespectral region as being particularly suitable for measuring the infra-red absorptive powers of the soil extract solutions for the purpose of my invention.

In conducting the absorption measurements, I use the well known infra-red apparatus set-up which comprises essentially an infra-red light source, such as a globar, or a Nernst lamp, an absorption cell, anda standard spectrometer having a thermocouple and a galvanometer operatively associated therewith.

To determine the infra-red absorption power of the various extract solutions, the solution to be tested is placed in an absorption cell and the energy of radiation for a certain selectedwavelength of infra-red light transmitted through the absorption cell is noted. The energy transmitted through a cell containing pure carbon tetrachloride solvent at the same wave-length setting is then measured. This latter measurement, which may be termed a blank, gives a value for the initial or incident radiation energy, while the former measurement determines the value of the transmitted radiation energy. From the values thus obtained, i. e., that of the transmitted and that of the incident radiation energy, a value related to the concentration of hydrocarbon substances in the extract solution is readily obtainable, since, according to Beers law, the concen-- tration of hydrocarbons in the extract solution is proportional to the amount of infra-red energy absorbed.

Since, as stated previously, the most significant absorption efiects produced by soil hydrocarbons are in the region of from 2.8 to 4.0 with particularly high absorption being consistently indicated at about 3.35,u, in determining the absorptive power of an extract solution, I may operate the spectrometer according to an of the following procedures.

1. I may operate the spectrometer so that a relatively narrow band of radiation, preferably including a wave-length which has been found to be highly absorbed by the soil hydrocarbons, for

example 3.35 will be transmitted through the absorption cell. The absorption values thus obtained for the different sample extract solutions may then be compared in any suitable manner, such as by plotting them as a function of sampling location in the field.

2. I may operate the spectrometer so as to permit a relatively wide band, or envelope, of infrared energy, said envelope including wave-lengths from say 2.8 to 4.0/L, to'be transmitted through the absorption cell, the amount of energy absorbed from this envelope of rays being taken as the absorption value of an extract solution.

3. I may make several measurements of the absorptive power of each solution at narrow band sett ngs, for example at intervals of say .05 l or .1 within the range of from 2.8 to 4.0 1.. By this method, to obtain the absorptive power of an extract solution, I plot the values for the absorptive powers determined at the different wavelength settings as a function of wave-length in this selected range. In this way I obtain a curve having a maximum approximately at 335 and minima on either side thereof. I connect these minimum values by a straight line and then determine the enclosed area (i. e. the areaenclosed by the curve and the straight line). This area may be determined conveniently by means of a planimeter, or it may be calculated in the well known manner of determining the area under a curve. This area I term the A value and it is representative of the hydrocarbon concentration of the extract solution. The A values for the different extract solutions made up from samples taken in a survey are then compared in relation to the sampling locations, as by plotting them as a function of sample location on a traverse, or by locating them on a map of the area under survey, in order that the petroleum deposit may be located.

In order that the method of the invention will be readily understood the manner of determining the A values in accordance with the third method given above is illustrated in Figure 1 of the drawings.

Referring to Figure l, the graph shows a curve values, expressed as the negative logarithm of the .ratio of the transmitted energy to the incident energy (i. e. -log I/Io), as ordinates, v. wavelength settings for the measurement of these values, as abscissae. It will be observed that the curve rises to a maximum value, or peak 2 at 3.35;, and that minima values 3 and 4 appear at each side of this peak at wave-lengths of 2.90 and 3.85,, respectively. The value of the hydrocarbon concentration in this sample is represented by th area A enclosed by the curve I and the straight line 5 drawn between the minima values 3 and 4. As stated hereinbefore this area, or A value, may be determined by the use of a planimeter, or it may be calculated by any of the well known methods, such as by counting the squares in the enclosed area. The A values thus determined for the difierent samples are taken and compared in the manner described herebelow in order to determine the location of an underlying pool.

It will be seen from Figure 1 that when the first of the methods mentioned is used, it would not be necessary to calculate the A values as just above described, since by that method only one absorption value, represented at the peak of the curve I would be determined for each solution. Although the first above method of ascertaining the absorptive power of the sample is relatively simple, as is method 2, when compared to the A value determination of method 3, I have found the latter method to be useful where a spectrometer of low resolving power is employed for the absorption measurements.

The absorption values for the different soil extract solutions, as determined by any one of the above outlined methods, may be compared or interpreted in several ways in relation to the sampling locations in order to obtain the desired information as to the location of a petroleum pool. For example, in a survey where the samples have been taken at spaced intervals along a traverse, or a number of traverses, the values for the samples in any one traverse may be plotted as a function of traverse distance. A plot of this type is illustrated in Figure 2 of the drawings.

' Referring to Figur 2, the graph shows a curve 1 derived by plotting the A values as determined for a series of samples taken along a traverse, as ordinates, against the traverse sampling distances, as abscissae. This particular survey was conducted over the Talco Oil Field in northeast Texas. In order to illustrate the relationship between the infra-red absorption values determined for the various soil samples and the position of the oil pool the curve obtained has been superimposed on a scale drawing in crosssection of this field taken along the traverse on which the sampling stations were located. As shown in the drawing, the numeral 8 represents the earths surface, while 9, H], H and I2 represent the different strata. The numerals l5 to 35 inclusive represent sampling stations set out at 1000 feet intervals along the traverse; The oil pool is shown at 36 while 31 represents a 45 fault plane running east and west across the field.

It will be observed that the curve I shows a distinct anomaly, indicated by the peak 38. The anomaly begins approximately at station 2|, reaches a maximum at station 25, and disappears beyond station 30. The background values for the infra-red absorptive powers of the extract solutions prepared from samples taken in, the

prospecting method, it is an advantageous toolarea extending away from the pool are shown by the portions 39 and 40 of the curve.

It will be noted that the anomaly extends to stations 28, 29 and 30, which are not directly over the pool, and that the peak 38 of the anomaly is displaced toward the right side of the pool, rather than appearing in the center thereof as we have generally found in other surveys. An interesting explanation for these phenomena is that they are due to the presence of the fault plane 31, which apparently acts to divert substances emanating from the pool, i. e. the hydrocarbon gases, which are related to the presence of the indicating hydrocarbons in the soil.

The values for the absorptive powers of the various extract solutions as determined by any of the methods described hereinabove are not absolute values, but relative only. For the purposes ofthe invention these relative values are sufficient since only differences in concentration need be measured in order to produce the indicating anomalies. However, if the absolute values of the hydrocarbon concentrations are desired to be obtained, the relative values may be readily converted to absolute values. To do this several standard extract solutions may be prepared, and the absorptive powers of the unknown extract solutions compared to these standards. A standard extract solution of any desired concentration may be conveniently prepared by extracting a relatively large soil sample, for example 5000 grams, evaporating off the solvent, weighing the waxy hydrocarbon residue and redissolving this residue in any known volume of solvent to give the desired soil hydrocarbon con-- centration.

The method of measuring the soil hydrocarbons: in accordance with my invention possesses a. number of advantages over the heretofore proposed methods. Thus, by its use large numbers: of samples can be readily analysed with a minimum expenditure of time. The manual operations involved in the method are simple as is the computation of results, in fact these steps may be made automatic and self-recording if desired. Also, the results obtained in a survey utilizing my method are more accurate and reliable since anomalies amounting to only a few parts per million of soil hydrocarbons are readily detectable thereby. Furthermore since my method measures only the carbon-hydrogen bonds present in the extract solution in each case, it is not susceptible to errors often incurred by other methods which measure the total amount of extracted materials.

Although in describing my invention I have mentioned that I generally prefer to collect the soil samples from points along a traverse in a survey area, it will be understood that the method of the invention embraces the use of any desired sampling scheme, such as sampling according to a grid or radial sampling pattern, or where scattered samples are taken and located on a map of the survey area. Also the invention embraces the use of methods wherein the samples are taken according to a vertical rather than a horizontal sampling plan in order to detect an underlying deposit, as I have found that the nearer the samples are to the pool the greater are the absorptive powers of the resulting extract solutions.

Although the method of my invention is capable of detecting anomalous indications of petroliferous deposits when used as an independentwhen used as a supplementary method c. 11- junction with other survey methods, such as, for example, the seismic or gravimeter methods. The method is especially useful in this latter connection in the detailing of areas where indications of structures have been found by geophysical methods.

Having now fully described my invention and the manner of operation thereof, what I claim as new and useful and wish to be protected by Letters Patent is:

I claim:

1. The method of geochemical prospecting for petroliferous deposits which comprises collecting a number of soil samples at spaced sampling locations in a prospect area, extracting from .said samples the hydrocarbon materials present therein by means of an organic solvent, passing infra-red rays of wave length between 2.8 and 4.0 microns through the extracted material dis- .solved in an organic solvent containing no carbon-hydrogen bonds and exhibiting a high trans- .mission toward infra-red radiation to deter- ;mine the infra-red absorptive abilities of said so- .lutions as a measure of the relative amounts of hydrocarbon materials originally present in said soil samples and comparing the absorption values :so determined in relation to the sampling point locations in the prospect area whereby indications as to the presence and location of a petroliferous deposit are obtained.

2. The method of exploring for petroleum deposits Which comprises taking samples of soil from spaced locations in a prospect area, extracting the hydrocarbons contained in said samples by means of an organic solvent, passing infra-red rays through the extracted material dissolved in an organic solvent containing no carhon-hydrogen bonds and exhibiting a high transmission toward infra-red radiation, observing the amount of infra-red energy absorbed by each of said solutions from a band of rays, saicl band including rays of wave-length from about 2.8a to 4.0 1, as a measure of the hydrocarbon content of the soil sample from which the extract was prepared and correlating the absorption values so determined in relation to the sampling locations in the prospect area, whereby indications as to the presence and location of a petroleum deposit are obtained.

3. The method of prospecting for buried petroliferous deposits which comprises collecting a number of soil samples from selected locations in .a prospect area, extracting the hydrocarbon contents of said samples by means of an organic solvent, passing infra-red rays through the extracted material dissolved in an organic solvent containing no carbon-hydrogen bonds and exhibiting a high transmission toward infra-red radiation, determining the amount of radiation .energy absorbed by each solution at a plurality of band settings within the spectral range of from 2.8/J- to 4.0/4, selecting from the determinations so made the highest absorption value for each solution as a measure of the hydrocarbons :contained in the sample from which said solution was prepared and correlating said selected values in relation to the sampling locations in the prospect area, whereby indications of the presence and location of a petroliferous deposit ,may be obtained.

4. In the method of geochemical prospecting for petroliferous deposits which comprises col- .lecting a number of soil samples from selected locations in a. prospect area, determining the amount of hydrocarbon constituents present iii said samples and correlating the results so obtained in relation to the respective sampling locations in the prospect area to obtain indications as to the presence and location of a petroliferous deposit, the improvement which comprises determining the amount of hydrocarbon constituents in the soil samples by extracting said samples with an organic solvent, passing infra-red rays through the extracted material dissolved in an organic solvent containing no carbon-hydrogen bonds and exhibiting a high transmission toward infra-red radiation, and measuring the amount of infra-red energy absorbed by said solutions from a band of rays of wavelength between 2.8 and 4.0 microns as a measure of the hydrocarbon contents of the respective soil samples.

5. The method of analyzing soil for minute quantities of hydrocarbon materials contained therein which comprises extracting a sample of said soil by means of an organic solvent to obtain an extract solution thereof, passing infrared rays of wave length between about 2.8 and 4.0 microns through the material extracted from the soil dissolved in an organic solvent containing no carbon-hydrogen bonds and exhibiting a high transmission toward infra-red radiation and observing the amount of infra-red energy absorbed by said extract solution as a measure of the concentration of hydrocarbons in said sample.

6. The method of exploring for petroleum deposits which comprises taking samples of soil from spaced locations in a prospect area, extracting the hydrocarbons contained in said samples by means of an organic solvent, passing infra-red rays through the extracted material dissolved in an organic solvent containing no carbon-hydrogen bonds and exhibiting a high transmission toward infra-red radiation, observing the amount of infra-red radiation absorbed from the rays having a wavelength of about 3.35 microns as a measure of the hydrocarbon content of the soil sample from which the extract was prepared and correlating the absorption values so determined in relation to the sampling locations in the prospect area, whereby indications as to the presence of a petroliferous deposit is obtained.

'7. The method of claim 4 in which said organic solvent through which the infra-red radiations are passed is carbon tetrachloride.

8. The method of claim 5 in which said organic solvent through which the infra-red radiations are passed is carbon tetrachloride.

NELSON P. STEVENS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,265,357 Demares Dec. 9, 1941 2,269,889 Blau Jan. 13, 1942 2,305,082 Hocott Dec. 15, 1942 2,311,151 Campbell Feb. 16, 1943 2,376,311 Hood May 15, 1945 OTHER REFERENCES Infra-Red Spectroscopy, Barnes et al., published in 1944 by Reinhold Publishing Com, 300 W. 42 St., New York, N. Y. 

1. THE METHOD OF GEOCHEMICAL PROSPECTING FOR PETROLIFEROUS DEPOSITS WHICH COMPRISES COLLECTING A NUMBER OF SOIL SAMPLES AT SPACED SAMPLING LOCATIONS IN A PROSPECT AREA, EXTRACTING FROM SAID SAMPLES THE HYDROCARBON MATERIALS PRESENT THEREIN BY MEANS OF AN ORGANIC SOLVENT, PASSING INFRA-RED RAYS OF WAVE LENGTH BETWEEN 2.8 AND 4.0 MICRONS THROUGH THE EXTRACTED MATERIAL DISSOLVED IN AN ORGANIC SOLVENT CONTAINING NO CARBON-HYDROGEN BONDS AND EXHIBITING A HIGH TRANSMISSION TOWARD INFRA-RED RADIATION TO DETERMINE THE INFRA-RED ABSORPTIVE ABILITIES OF SAID SOLUTIONS AS A MEASURE OF THE RELATIVE AMOUNTS OF HYDROCARBON MATERIALS ORIGINALLY PRESENT IN SAID 